Leakage current is a type of non-functional current that flows through an electrical device, even when the power to it is turned off. Sometimes referred to as "vampire power," leakage current contributes to wasted energy and can pose a personal safety hazard.
Several types of leakage current have been defined: Earth, Enclosure, and Patient. Patient leakage current flows from the applied part of a medical device through a patient to ground. Patient leakage current results in a waste of energy and can also be dangerous to human life, as the electrical current flows through a person.
"Medical Electrical Equipment—Part 1: General Requirements for Safety and Essential Performance" (IEC 60601-1) is a standard that concerns the safety and performance of medical electrical equipment. IEC 60601-1 power supply requirements outline the necessary guidelines medical devices need to meet to ensure patient safety.
It defines the maximum leakage allowed from an applied part to reduce the risk of an electrical shock. Protecting against shock is particularly important for patients, who might have weakened immune systems and are at an increased risk of physical harm from an electrical current.
Classification of Applied Parts
An applied part is a piece of medical equipment that comes into contact with a patient. This can be in the form of a patient touching the part of the component otherwise making contact with the patient. In some instances, the part needs to connect with or touch the patient to function properly.
The IEC 60601-1 standard classifies applied parts into three categories based on the type of contact they have with a patient and the type of device. Those medical equipment types are B, BF, and CF. These medical equipment classifications provide valuable device information to medical professionals and device engineers alike.
Body or Type B parts are usually not conductive. Type B is the least stringent of the three classifications, as the part is often something that a patient can let go of easily. Examples of Type B applied parts include magnetic resonance imaging (MRI) scanners, lasers, phototherapy equipment, and hospital beds. Type B parts can be connected to ground.
Body Floating (BF)
Body floating or Type BF parts often do have some conductive contact with a patient, which is either long-term or medium contact. Examples of devices that fall under the Type BF classification include ultrasound equipment, incubators and blood pressure cuffs. As the name suggests, Type BF parts are floating and do not have connections with ground.
Cardiac Floating (CF)
Cardiac floating requirements are the strictest of the applied parts classifications. Any parts that come into contact with the heart or bloodstream are classified as Type CF. Examples include dialysis and surgical equipment. In the case of BF classification vs. CF classification, both categories share various similarities. Like Type BF, Type CF is floating and doesn't connect to earth ground.
Each of the three classifications can also be rated as defibrillation proof. A medical device may require defibrillation-proof applied parts if it will need to be used on a patient when a defibrillator is applied.
How Each Classification Protects Against Shock
Under the standard, a medical device needs to offer two means of protection (MOP) to keep applied parts from delivering too high of a voltage or current. The MOP is the isolation protection between circuitry and equipment that might contact the device. Examples of isolation protection include protective earth connections, electrical insulation, and creepage distances.
MOP is divided into two categories: means of patient protection (MOPP) and means of operator protection (MOOP). Types BF and CF require the greatest amount of MOPP.
Leakage Current Allowances
One factor that determines how severe the harm caused by an electrical shock can be is the bodily location where the shock occurs. If a person has limited contact with an applied part, the risk of severe harm is considerably lower than the injury that could occur if the part is in direct contact with their heart or bloodstream. For that reason, each type has different leakage current allowances, based on the standard. The allowances also differ based on whether the current is AC or DC.
Benefits of Choosing the Correct Power Supply
Power supplies are not medical devices and are separate from applied parts. However, the standard does describe the leakage current and isolation levels a power supply needs to meet to qualify. The patient leakage limit for a power supply depends on the type of applied part that will be in use. Per the standard, the limits are:
- Type B: 100µA under normal conditions, 500µA under single-fault conditions.
- Type BF: 100µA under normal conditions, 500µA under single-fault conditions.
- Type CF: 10µA under normal conditions, 50µA under single-fault conditions.
Since the risk of physical harm to a person is greatest when an applied part is in contact with their heart or bloodstream, the limits are considerably lower for power supplies in parts that fall under the Type CF classification. The benefits of using a CF power supply can be significant if an engineer is looking for a model that meets more stringent requirements. Additionally, medical professionals can ensure patient safety by using Type CF devices outfitted with the appropriate CF power supplies.
A power supply must also meet the isolation requirements established by the standard to reduce leakage. Those conditions are:
- Type B: Input to Output Isolation: 4,000 volts alternating current (VAC) or 2 MOPP, Input to Ground Isolation: 1,500 VAC or 1 MOPP, Output to Ground Isolation: 500 VAC.
- Type BF: Input to Output Isolation: 4,000 VAC or 2 MOPP, Input to Ground Isolation: 1,500 VAC or 1 MOPP, Output to Ground Isolation: 1,500 VAC.
- Type CF: Input to Output Isolation: 4,000 VAC or 2 MOPP, Input to Ground Isolation: 1,500 VAC or 1 MOPP, Output to Ground Isolation: 1,500 VAC.
BF Classification vs. CF Classification: Benefits of Using CF Power Supply
A medical device's power supply design must comply with isolation, leakage, and ground requirements. However, many of the AC/DC power supplies approved for use in the medical setting don't meet the patient leakage current requirements or lack the isolation required from output to ground. These power supplies often aren't appropriate for direct connection with patients.
One option is to use a medically approved DC/DC converter for a second isolation stage. This option can reduce the patient leakage current to a μA in the single digits, often around 2μA, which is appropriate for both BF and CF classifications. However, an issue arises in that DC/DC converters may lack the power required for many medical devices such as electrosurgical tools, dialysis machines, and incubators. Additionally, this architecture may add significant cost and needed size to the design.
Another option to achieve both BF & CF classification is to use an Isolation Transformer in between the mains AC input and the power supply. The biggest drawback of this option is that the size of the Isolation Transformer can be large and heavy depending on the power level needed, often larger than the power supply itself. The introduction of an Isolation Transformer will also introduce undesirable efficiency loss.
The most optimal solution for BF and CF Medical devices is a power supply specifically designed and NRTL certified to have the required spacing, patient leakage current, and isolation. Although BF-rated power supplies can be sufficient in some cases, the most life-critical Medical device applications should use a CF-rated power supply for maximum patient safety.
Learn More About Astrodyne TDI's Medical Grade Power Supplies, Including CF options
Medical devices need dependable power supplies to protect and provide lifesaving care to patients. Astrodyne TDI's portfolio of medical-grade power supplies includes ultra-low leakage products with AC/DC capabilities. Many of our power supplies meet Type CF requirements for an optimal system architecture to meet patient safety requirements.
If you're looking for a power supply that meets body floating (BF) or cardiac floating (CF) requirements for medical equipment, browse our inventory of medical power supplies or contact us today to learn more.